MOSCOW – Three Ukrainian military officers on the Crimean peninsula remained missing Sunday and were believed to be held by Russian forces, a Ukrainian official said, as the Russians continued to seek full control of the peninsula’s military sites.

Col. Yuli Mamchur, the commander of an air force unit stationed in the town of Lubimovka near Sevastopol, had been missing more than 24 hours Sunday evening after he was taken by Russian troops, Alexei Mazepa, a Ukrainian Defense Ministry spokesman, told The Times.

The deputy commander of Ukraine's navy, Ihor Voronchenko, was also unaccounted for and believed held by the Russians after he left navy headquarters in Sevastopol, which is under Russian control, Mazepa said.

Russian forces occupied the Ukrainian peninsula late last month, surrounding most Ukrainian military bases and units. Russian President Vladimir Putin declared last week that his country was taking over Crimea following a disputed referendum among the region’s voters, a majority of whom speak Russian, backing annexation.

Since then, Russian troops and pro-Russian militia have been seizing military sites still in Ukrainian hands. The Russian Defense Ministry press service reported Sunday that its nation’s flag has been raised and the Russian national anthem sung at the sites of 189 Ukrainian military units deployed in Crimea.

On Saturday, Russian troops used six armored vehicles to break through the fence around Mamchur's air force unit and then fired into the air, subduing the base's personnel. One Ukrainian officer reportedly was injured.

Mazepa said of Mamchur: “To save lives he refrained from ordering armed resistance on Saturday when the Russian troops and local [pro-Moscow] militia stormed his unit.”

Some Ukrainian units have complained that they were given no instructions from Kiev, the Ukrainian capital, in the face of Russian aggression. Mazepa said it was possible his nation’s leadership was delaying in hopes of action this coming week by the United Nations General Assembly.

“It is a possibility that the U.N. General Assembly may declare Crimea a demilitarized zone and demand the withdrawal of both Russian and Ukrainian troops from it simultaneously,” Mazepa said. “If there are no Ukrainian troops by that time left in Crimea, this question can't be considered. This may explain why Russians are doing their utmost to capture as many of Ukraine's remaining bases as they can and raise their flag over them.”

At the last remaining land base in the city of Feodosiya where Ukraine marines were stationed, some of the Ukrainians raised a Russian flag over one building Sunday while marines elsewhere at the site attached a Ukrainian flag to balloons and floated it about 100 yards overhead, Mazepa said.

At least two Ukraine army units and three naval vessels also were still resisting the Russian troops, said Dmitry Tymchuk, a Ukrainian defense expert.

“The situation is very volatile and Russian pressure is growing by the minute,” Tymchuk, head of Kiev-based Center for Military and Political Research, said in a phone interview. “The way the Russians move they can capture these units and vessels within the next few hours.”

Ukraine's worst unrest since the 1991 collapse of the Soviet Union was sparked by ousted President Viktor Yanukovich's decision to scrap a deal with the European Union in favor of closer ties with Russia.

Russian President Vladimir Putin said Wednesday that he had pulled back troops from the border with Ukraine and called on Kremlin-allied separatists to delay a referendum on whether Ukraine's regions should have a greater degree of autonomy.

At least 14 people were killed and scores were wounded as government forces seeking to win back this eastern Ukrainian city from pro-Russia separatists ran into an ambush Monday set up by the rebel gunmen.

A crowd of more than 2,000 pro-Russia separatists on Sunday night seized one of the last key official sites in the eastern Ukraine city of Donetsk still in the hands of the country’s interim government.

Armed clashes left scores of people dead in Ukraine on Friday, as President Obama and German Chancellor Angela Merkel vowed to impose stiffer economic sanctions on Russia if it disrupts Ukraine's upcoming election.